This invention relates to an automatic dishwasher equipped with a pumping system having a single reversible drag motor.
Dishwashers equipped with a pumping system are known. For example, EP-A-O 268 835 discloses a single reversible electrical motor having a drag shaft on which are fitted the impellers of a circulation pump and a discharge pump. The two pumps are activated alternately, according to the direction of rotation of the motor shaft.
The circulation pump is preferably of a centrifugal type, while the discharge pump is of the peripheral downflow type. In particular, the impeller of the discharge pump is set in a pumping chamber of substantially circular section defined by a lateral surface having an aspiration inlet and a delivery outlet. The aspiration inlet and the delivery outlet, respectively connected to the washing basin of the machine and to an external discharge conduit, are substantially adjacent but are separated over an angular extent by a portion of the lateral surface of the chamber which has a reduced inside diameter and is normally designated as the "tooth" of the pump.
As is known, when the delivery outlet of the discharge pump has a watertight connection with the hydraulic discharge conduit, the pump is generally provided with an air intake suitable for placing the delivery outlet in communication with the atmosphere through the washing basin, for example. This air intake has the function of preventing the discharge pump, when it is activated in the reverse rotation direction, from aspirating water from the external hydraulic system through the delivery outlet itself.
Also, during a discharge phase of the dishwasher, when the peripheral downflow type of discharge pump is activated in the forward direction of rotation, part of the water flowing out through the delivery outlet is expelled through the air intake. This amount of water collects on the bottom of the washing basin and returns to the aspiration inlet of the discharge pump with a certain delay, thus constituting an undesirable residue which limits the operations of that pump.
Also, when the discharge pump is activated in the forward direction of rotation, the air intake may easily be choked by the dirty water under pressure.